1. Field of the Invention
This invention relates generally to semiconductor processing, and more particularly to methods of probing a workpiece using hot-electron induced photon emission.
2. Description of the Related Art
Fault-isolation techniques are critical to the development and manufacture of large-scale integrated circuits such as microprocessors. As the numbers of devices per integrated circuit have continued to climb and the sizes of those devices continued to shrink, methods have been developed to probe the operation of integrated circuits at the device level.
Electron beam micro probing has been used for a number of years as a means of analyzing electrical wave forms generated by the various microscopic circuit structures in an integrated circuit. An electron beam (xe2x80x9ce-beamxe2x80x9d) micro probe is a particularized type of electron microscope that is designed to provide a visual image of the circuit structures on an integrated circuit. E-beams are specifically focused at targeted circuit structures on the integrated circuit and the reaction of the circuit structures to the directed e-beams are sensed by the microscope. Actual electrical test patterns can be used to stimulate the integrated circuit in various ways during the scanning. This is normally accomplished by mounting an integrated circuit on a test board. As with other types of electron microscopy, high vacuum conditions are required for e-beam micro probing.
One method proposed for providing improved waveform probing over conventional electron beam probing has been coined Picosecond Imaging Circuit Analysis or PICA for short. PICA measures time-dependent hot carrier induced light emission from an integrated circuit (IC) both spatially and temporally, thus enabling failure analysis and timing evaluation of a device. Hot electron light emission is generated as a short duration pulse coincident with the normal logic state switching of MOS circuits. This emission can be readily observed and used to directly measure the propagation of high-speed signals through the individual gates. The technique is useful in that non-invasive diagnostics of fully functional MOS devices may be performed.
In one conventional PICA approach, an imaging micro-channel plate photo-multiplier tube (MCP-PMT) is used to detect to the photons. Within the field of view of the objective, the technique allows for parallel acquisition of time resolved emission from many nodes at once. Unfortunately, a typical conventional MCP-PMT detector has low quantum efficiency, especially in the near infrared region. In particular, the detector loses virtually all sensitivity for wavelengths above 900 nm. For acquisition of photon emission from the backside of silicon substrates, this has proved problematic. As a result of the spectral characteristics of hot carrier emission and the optical transmission characteristics of doped silicon, most backside transmitted photons will be in the 900 to 1,500 nm range. Thus, the typical MCP-PMT will detect few of the available photons. This can lead to lengthy acquisition times.
Superconducting hot-electron photodetectors (xe2x80x9cSHEPxe2x80x9d) are being proposed as another type of photodetector for hot-electron emission probing of integrated circuits, whether by PICA or other technique. Conventional SHEPs are operated in a resistive mode. When a hot-electron emission photon is absorbed, the superconducting state of the SHEP is temporarily halted, at least in a localized area This loss of superconductivity is sensed as a sudden increase in resistivity. The departure from a superconductive state is thought to be the result of the breaking of Cooper pairs within the lattice of the SHEP.
Conventional SHEPs can exhibit a limited quantum efficiency. The fallout of limited quantum efficiency is the possibility of the SHEP not detecting certain circuit switching events. A missed switching event may be inadvertently interpreted as a fault in the circuit device rather than a missed photon event. To be able to distinguish the difference with confidence, the testing engineer will need to test the circuit element for time periods sufficient to make up for the lack of quantum efficiency. This may entail long testing periods.
The present invention is directed to overcoming or reducing the effects of one or more of the foregoing disadvantages.
In accordance with one aspect of the present invention, a method of examining a workpiece is provided that includes directing a first photon at a photodetector at a first known time and stimulating a circuit device of the workpiece at a second known time to produce a condition in the circuit device conducive to photon emission. At least one photon emitted by the circuit device in response to the stimulation is detected. The first photon increases the quantum efficiency of the photodetector in detecting the at least one photon. The detection of the at least one photon relative to the first known time and the second known time is time correlated to temporally distinguish the first photon and the at least one photon and to temporally correlate the stimulation of the circuit device to the detection of the at least one photon.
In accordance with another aspect of the present invention, a method of examining a workpiece is provided that includes directing a first photon at a superconducting hot-electron photodetector at a first known time and stimulating a circuit device of the workpiece at a second known time to cause the circuit device to go into saturation. At least one photon emitted by the circuit device in response to the stimulation is detected. The first photon increases the quantum efficiency of the superconducting hot-electron photodetector in detecting the at least one photon. The detection of the at least one photon relative to the first known time and the second known time is time correlated to temporally distinguish the first photon and the at least one photon and to temporally correlate the stimulation of the circuit device to the detection of the at least one photon.
In accordance with another aspect of the present invention, a method of examining a workpiece is provided that includes directing a plurality of photons at a superconducting hot-electron photodetector at corresponding known times. A transistor on the workpiece is stimulated at a second known time to cause the transistor to go into saturation. At least one photon emitted by the transistor in response to the stimulation is detected. The first photon increases the quantum efficiency of the superconducting hot-electron photodetector in detecting the at least one photon. The detection of the at least one photon relative to the first known time and the second known time is time correlated to temporally distinguish the first photon and the at least one photon and to temporally correlate the stimulation of the circuit device to the detection of the at least one photon.